Sauron
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Thiourea -> S-Me isothiourea -> MeSCN
The preparation of methyl thiocyanate by rge rgernolytic reaction of S-methyl isothiourea hydrochloride is documented by Arndt in Ber. 40, referenced
from Org.Syn's paper on the prep of the hemisulfate of the same compound from thiourea and dimethyl aulfate.
It is easy to convert the hemisulfate to the HCl salt by treating thw hemisulfate with two equivalents of BaCl2 in water.
After evaporation the hydrochloride (mp 59 C) is heated to 160-200 C at wgich temperature it fragments to NH4Cl and MeSCN. From a Kg of the
hydrochloride more than 550 g MeSCN can be distilled.
S-methyl isothiourea hemisulfate is commercially available and inexpensive.
If one wants to prepare his own, thiourea (very inexpensive) can be methylated with two mols of methylsulfuric acid rather than one mol DMS.
Barium chloride 2H2O is readily available, and can also be prepared from barium hydroxide or carbonate and hydrochloric acid.
This process is substantially cheaper than KSCN + DMS or MeI or MeBr. Methylsulfuric acid does not work with KSCN. So DMS must be used on an
equimo;ar basos.
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Ullmann
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| Quote: | If one wants to prepare his own, thiourea (very inexpensive) can be methylated with two mols of methylsulfuric acid rather than one mol DMS.
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Do you have a write-up for this? It would be interesting as a way to obtain mesyl chloride by chlorination of methylisothiourea and then methyl
methanesulfonate by basic methanolysis...
It is strange methylsulfuric cannot methylate KSCN but can on the thiourea isn't it?
Much appreciated if you could give an experimental proof on this particular method of synthesis of methylisothiourea
Regards,
Dr U
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Sauron
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Just go to www.orgsyn.org and search on S-methyl isothiourea hemisulfate.
That prep is based on the German paper by Arndt cited therein. Thge same paper details the conversion to hydrochloride, and thermolysis to MeSCN.
No it is not strange that methyl hydrogen sulfate will methylate thiourea but not KSCN. It is all a matter of pKa.
For example, dimethyl sulfate methylates 2 eq of KCN. HCN is a weak acid, KCN therefore a strong base. The first mol of KCN is methylated
exothermically by DMS which on the process turns into methyl hydrogen sulfate. At that point the reaction stops, until external heat is applied. That
drives the methylation of the other mol.
The product is MeCN acetonitrile.
Exactly the same thing happens with thiourea. Thiourea is in tautomerism with isothiourea which is protonated and has a formal +1 charge on the -SH.
That is a weak acid.
Since we can see that methyl hydrogen sulfate (methylsulfuric acid) succesfully methylates isothiourea in the second phase of the Arndt reaction, it
is entirely reasonable to believe that it can replace DMS in the first phase as well, as lomg as external heating (reflux in aqueous soln) is applied
from start. Do you follow?
Now, KSCN is a weak base and we know that HSCN is a very strong acid. DMS will only methylate 1 eq of KSCN (which is does exothermically at RT and
quantitatively).
Maybe you can drive MeHSO$ to methylate KSCN in a sealed tube or autoclave but not at 1 atm under the bp of MeSCN.
[Edited on 3-7-2009 by Sauron]
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Sauron
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Another example:
KNO2 is succesfully methylated by DMS, but only 1 eq not 2,
This means MeHSO4 will not work at least at 1 atm.
Product is MeNO2 nitromethane.
See Walden citation in Nicodem's post in Iodomethane thread. Also the Monatsc. citation same post, I forget the name of the author. Can you read
German? Anyway you can read the equations. The full txts are in Refs.
Is nitrous acid a weak acid or a strong acid?
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Nicodem
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Quote: Originally posted by Sauron  |
No it is not strange that methyl hydrogen sulfate will methylate thiourea but not KSCN. It is all a matter of pKa. |
The pKa has nothing to do with it when you are methylating with MeOSO3K or other alkali methylsulfates (it is the nucleophilicity of the reagent and
its thermal stability to the reaction conditions that matters). The pKa of the nucleophile is only important if you want to methylate with MeOSO3H
which is a strong acid thus inactivating basic nucleophiles. For example, nitrite salts can be methylated with MeOSO3K (there is even a dedicated
thread on the forum), but they can not be methylated by MeOSO3H because as an acid it just decomposes nitrites to nitrous oxides. Similarly KCN can
not be methylated with methylsulfuric acid but it can be methylated with the MeOSO3K (according to the reference you found where the MeOSO3K formed
from (MeO)2SO2 + KCN reacts with another equivalent of KCN). KSCN obviously can react with methylsulfates (I even provided one such reference in the
methyl iodide thread), but I do not believe it can be methylated with methylsulfuric acid.
In reality there are only very few cases where methylsulfuric acid can be used for methylation, because there are only a limited number of
nucleophiles that are not too basic for it. It can work with H2O (which leads to MeOH and H2SO4), with chloride, bromides and iodides (Persona gave
his account of iodide methylation with MeOSO3H in the methyl iodide thread), potentially with some thiols, and a few others.
Methylsulfates (which are not acidic) have way more general applicabilities, the only problem is their reduced electrophilicity in comparison with
MeOSO3H, but this can be overcome by heat.
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unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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Sauron
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Intwo of the references you cited in the Iodomethane thread KNO2 and KSCN r4eacted with Me2SO4 but the reaction stopped when DMS was exhausted;
potassium methyl sulfate did not react with additional salt. No MeHSO4 waspresent. I misspoke above.
Two equivalents KCN reacted with one of Me2SO4 so MeSO3K does methylate KCN.
I made no mention of halides.
I have not fully translated the two refs on Ca(MeSO3)2 but I have seen no yield data thus far, The papers aee 1840s.
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